Polarizing plate and optical display apparatus

The present application claims priority to and the benefit of Korean Patent Application No. 10-2023-0027991, filed on Mar. 2, 2023 in the Korean Intellectual Property Office, the entire disclosure of which is incorporated herein by reference.

Aspects of embodiments of the present invention relate to a polarizing plate and an optical display apparatus.

A light emitting diode display including organic light emitting diodes or a liquid crystal display is provided with a polarizing plate in order to improve optical characteristics. In the polarizing plate, a polarizer includes a polyvinyl alcohol based film dyed with dichroic pigments and stretched. The polarizing plate is required to have durability under high temperature and high humidity conditions.

The polarizer may include a retardation layer to provide an optical compensation function. Although the retardation layer may be a single layer, the retardation layer can have better optical properties when present in a multilayer structure.

The background technique of the present invention is disclosed in Korean Patent Laid-open Publication No. 10-2013-0103595 and the like.

According to an aspect of embodiments of the present invention, a polarizing plate that exhibits good durability even after being left under high temperature and high humidity conditions or at high temperature for a long period of time is provided.

According to another aspect of embodiments of the present invention, a polarizing plate that has good frontal contrast ratio, suppresses light leakage at a lateral side through reduction in lateral luminance in a black mode, and reduces color shift at the lateral side is provided.

According to an aspect of one or more embodiments of the present invention, a polarizing plate is provided.

According to one or more embodiments, a polarizing plate includes: a polarizer; and a stack of retardation layers on a surface of the polarizer, wherein the stack of retardation layers includes a second retardation layer and a first retardation layer stacked on the polarizer in sequence from the surface of the polarizer, the second retardation layer being a non-liquid crystal layer, and wherein the stack of retardation layers has an out-of-plane retardation of −40 nm or more to less than 0 nm at a wavelength of 550 nm and satisfies Relation 1:

where Rth(450) and Rth(550) are out-of-plane retardations (unit: nm) of the stack of retardation layers at wavelengths of 450 nm and 550 nm, respectively.

According to another aspect of one or more embodiments of the present invention, an optical display apparatus is provided.

In one or more embodiments, an optical display apparatus includes the polarizing plate according to an embodiment of the present invention.

Embodiments of the present invention provide a polarizing plate that has good durability even after being left under high temperature and high humidity conditions or at high temperature for a long period of time.

Further, embodiments of the present invention provide a polarizing plate that has good frontal contrast ratio, suppresses light leakage at a lateral side through reduction in lateral luminance in a black mode, and reduces color shift at the lateral side.

Herein, some example embodiments of the present invention will be described in further detail with reference to the accompanying drawings such that the present invention can be implemented by those skilled in the art. However, it is to be understood that the present invention may be realized in various ways and is not limited to the following embodiments. Although lengths, thicknesses, or widths of various components may be exaggerated in the drawings for clarity of description of the present invention, the present invention is not limited thereto. Like components are denoted by like reference numerals throughout the specification.

The terminology used herein is for the purpose of describing example embodiments and is not intended to limit the present invention. As used herein, the singular forms “a,” “an,” and “the” are intended to include the plural forms as well, unless the context clearly indicates otherwise.

Herein, spatially relative terms, such as “upper” and “lower,” are defined with reference to the accompanying drawings. Thus, it is to be understood that “upper surface” can be used interchangeably with “lower surface” and vice versa. When an element is referred to as being formed “directly on,” “immediately on,” or “to directly adjoin” another element, there are no intervening element(s) therebetween.

As used herein, “in-plane retardation (Re),” “out-of-plane retardation (Rth),” and “degree of biaxiality (NZ)” are represented by Equations A, B, and C, respectively:

where nx, ny, and nz are the indexes of refraction of an optical element in the slow-axis direction, the fast-axis direction, and the thickness direction of the optical element at a measurement wavelength, respectively, and d is the thickness (unit: nm) of the optical element.

In Equations A to C, “optical element” may be a first retardation layer, a second retardation layer, a protective layer, or a stack of retardation layers. In Equations A to C, the measurement wavelength may be 450 nm, 550 nm, or 650 nm.

Herein, “(meth)acryl” refers to acryl and/or methacryl.

Herein, “water vapor transmission rate” means a value measured, for example, at 23° C. and at 99% RH (relative humidity) to 100% RH. The water vapor transmission rate may be measured using a measurement device (PERMATRAN-W, MODEL 700). A sample for measurement of the water vapor transmission rate may be prepared by cutting a retardation layer into a specimen having a size of 10 cm×10 cm (length×width), followed by placing the specimen in the measurement device. Herein, “water vapor transmission rate” of a protective layer may also be measured by the same method as above.

Herein, “glass transition temperature” of a second retardation layer may be measured by differential scanning calorimetry (DSC).

Herein, wavelength dispersion values of in-plane retardation at wavelengths of 450 nm, 550 nm, and 650 nm are Re(450)/Re(550), Re(550)/Re(550), and Re(650)/Re(550), respectively.

Herein, wavelength dispersion values of out-of-plane retardation at wavelengths of 450 nm, 550 nm, and 650 nm are Rth(450)/Rth(550), Rth(550)/Rth(550), Rth(650)/Rth(550), respectively.

As used herein to represent a specific numerical range, “X to Y” means a value greater than or equal to X and less than or equal to Y (X≤ and ≤Y).

Generally, a typical polarizing plate includes a polarizer including a polyvinyl alcohol based film dyed with dichroic pigments and stretched. The polarizer can suffer from elution of dichroic pigments when left under high temperature and high humidity conditions for a long period of time. The eluted dichroic pigments can contaminate an optical display panel, thereby causing deterioration in reliability of an optical display apparatus and clarity of screen quality thereof.

By contrast, a polarizing plate according to one or more embodiments of the present invention has good durability even after being left under high temperature and high humidity conditions for a long period of time. In an embodiment, the polarizing plate exhibits very low variation in light transmittance and polarization degree and has a trace amount of dichroic pigments or no dichroic pigments eluted therefrom after being left at 60° C. and 95% RH for 250 hours or more. For example, the dichroic pigments may be iodine and the like. In this regard, the polarizing plate includes a non-liquid crystal layer as a second retardation layer described below. Here, “elution of dichroic pigments” can be confirmed by evaluating the degree of iodine discoloration through observation of an edge of a polarizing plate after the polarizing plate is left in a chamber under high temperature and high humidity conditions (60° C. and 95% RH) for 250 hours.

The polarizing plate according to one or more embodiments provides an effect of increasing a frontal contrast ratio. In addition, the polarizing plate according to one or more embodiments reduces light leakage and color shift at a lateral side while securing good clarity on a black screen. Further, the polarizing plate according to one or more embodiments provides good viewing angle. In one or more embodiments, the polarizing plate includes: a polarizer; and a stack of retardation layers formed on a surface of the polarizer, wherein the stack of retardation layers includes a second retardation layer and a first retardation stacked on the polarizer in sequence from the surface of the polarizer, the second retardation layer being a non-liquid crystal layer, and wherein the stack of retardation layers has an out-of-plane retardation of −40 nm or more to less than 0 nm at a wavelength of 550 nm and satisfies the following Relation 1:

where Rth(450) and Rth(550) are out-of-plane retardations (unit: nm) of the stack of retardation layers at wavelengths of 450 nm and 550 nm, respectively.

As described below, the stack of retardation layers may be disposed between the polarizer and an optical display panel. To achieve improvement in durability under high temperature and high humidity conditions or at high temperature, the stack of retardation layers includes a non-liquid crystal layer as the second retardation layer and is formed to a suitable thickness instead of indefinitely increasing the thickness of the polarizing plate, thereby improving durability and frontal contrast ratio even under high temperature and high humidity conditions or at high temperature, and securing good visibility and good clarity on a black screen while reducing light leakage and color shift at a lateral side.

The polarizing plate according to one or more embodiments may be used as a viewer-side polarizing plate or a light source-side polarizing plate in a lateral electric field liquid crystal type optical display apparatus, for example, an IPS or FFS-mode liquid crystal display. In addition, the polarizing plate according to one or more embodiments may be used as an antireflection polarizing plate in a light emitting diode display, such as an OLED and the like. Here, “viewer-side polarizing plate” is a polarizing plate disposed to receive light emitted from a liquid crystal panel.

Next, the polarizing plate according to an embodiment of the invention will be described.

Stack of Retardation Layers

The stack of retardation layers is disposed between the polarizer and an optical display panel. The stack of retardation layers suppresses or prevents elution of dichroic pigments and variation in degree of polarization and light transmittance after the polarizing plate is left under high temperature and high humidity conditions or at high temperature for a long period of time. The stack of retardation layers increases frontal contrast ratio and provides good visibility and good clarity on a black screen while reducing light leakage and color shift at a lateral side.

The stack of retardation layers may be disposed on a light exit surface of the polarizer (for a light source-side polarizing plate) with respect to internal light, or may be disposed on a light incidence surface of the polarizer (for a viewer-side polarizing plate) with respect to internal light. Here, “internal light” may be light emitted from a backlight unit.

The stack of retardation layers includes the second retardation layer and the first retardation layer stacked in sequence from the polarizer.

In an embodiment, the second retardation layer may be directly formed on the first retardation layer. Here, “directly formed” means that no adhesive layer or bonding layer is formed between the first retardation layer and the second retardation layer. In another embodiment, the second retardation layer may be formed on the first retardation layer via an adhesive layer, for example, a pressure-sensitive adhesive (PSA) layer. When the second retardation layer is formed directly on the first retardation layer, or via the adhesive layer, the stack of retardation layers satisfies all retardation characteristics described below.

The stack of retardation layers has an out-of-plane retardation of −40 nm or more to less than 0 nm at a wavelength of 550 nm and satisfies the following Relation 1. As a result, the stack of retardation layers can easily increase frontal contrast ratio and provides good visibility and good clarity on a black screen while reducing light leakage and color shift at a lateral side.

where Rth(450) and Rth(550) are out-of-plane retardations (unit: nm) of the stack of retardation layers at wavelengths of 450 nm and 550 nm, respectively.

For example, the stack of retardation layers may have an Rth(550) value of −40, −39, −38, −37, −36, −35, −34, −33, −32, −31, −30, −29, −28, −27, −26, −25, −24, −23, −22, −21, −20, −19, −18, −17, −16, −15, −14, −13, −12, −11, −10, −10, −9, −8, −7, −6, −5, −4, −3, −2, −1 or 0 nm. In an embodiment, the stack of retardation layers may have an Rth(550) value of −25 nm to −5 nm, −25 nm to −10 nm, or −20 nm to −10 nm.

For example, the stack of retardation layers may have an Rth(450)/Rth(550) value of 1.1, 1.11, 1.12, 1.13, 1.14, 1.15, 1.16, 1.17, 1.18, 1.19, 1.2, 1.21, 1.22, 1.23, 1.24, 1.25, 1.26, 1.27, 1.28, 1.29, 1.3, 1.31, 1.32, 1.33, 1.34, 1.35, 1.36, 1.37, 1.38, 1.39, 1.4, 1.41, 1.42, 1.43, 1.44, 1.45, 1.46, 1.47, 1.48, 1.49, or 1.5. In an embodiment, the stack of retardation layers may have an Rth(450)/Rth(550) value of 1.15 to 1.5, and, in an embodiment, 1.2 to 1.4.

In an embodiment, the stack of retardation layers may have a negative Rth(450) value as in Rth(550). For example, the stack of retardation layers may have an Rth(450) value of −20 to −5 nm, for example, −20, −19, −18, −17, −16, −15, −14, −13, −12, −11, −10, −9, −8, −7, −6 or −5 nm, and, in an embodiment, −12 nm to −7 nm. Within this range, the stack of retardation layers can easily satisfy Relation 1.

The stack of retardation layers may further satisfy the following Relation 2. As a result, the polarizing plate can easily increase frontal contrast ratio and provides good visibility and good clarity on a black screen while reducing light leakage and color shift at a lateral side.

where Rth(650) and Rth(550) are out-of-plane retardations (unit: nm) of the stack of retardation layers at wavelengths of 650 nm and 550 nm, respectively.

For example, the stack of retardation layers may have an Rth(650)/Rth(550) value of 0.1, 0.2, 0.3, 0.4, 0.5, 0.6, 0.7, 0.8, 0.9, or 1.0. In an embodiment, the stack of retardation layers may have an Rth(650)/Rth(550) value of 0.2 to 1.0, and, in an embodiment, 0.5 to 0.9.

In an embodiment, the stack of retardation layers may have a negative Rth(650) value as in Rth(550). For example, the stack of retardation layers may have an Rth(650) value of −20 to −1 nm, for example, −20, −19, −18, −17, −16, −15, −14, −13, −12, −11, −10, −9, −8, −7, −6, −5, −4, −3, −2 or −1 nm, and, in an embodiment, −13 nm to −5 nm. Within this range, the stack of retardation layers can easily satisfy Relation 2.

The stack of retardation layers exhibits different wavelength dispersion of the out-of-plane retardation depending upon wavelength from wavelength dispersion of the in-plane retardation.

Referring to, the stack of retardation layers exhibits positive wavelength dispersion with respect to in-plane retardation and out-of-plane retardation depending upon wavelengths. However, it can be seen that wavelength dispersion of the out-of-plane retardation depending upon wavelengths is different than wavelength dispersion of the in-plane retardation depending upon wavelengths.

In an embodiment, the stack of retardation layers may satisfy the following Relations 3 and 4. As a result, the polarizing plate can easily realize the effects of the present invention.